1. Field of the Invention
The present invention relates to a symbol information reading apparatus for reading symbol; information such as bar codes.
2. Description of the Related Art
With the recent remarkable spread of POS (Point of Sales) information management systems, bar codes have become familiar to the public. A bar code is a symbol consisting of bars and spaces combined in parallel to form a single bar code character. If necessary, a character group including check digits is added in parallel to the bar code character. In addition, predetermined patterns, such as start and stop characters, are attached before and after the bar code character or the combination of the bar code character and the predetermined patterns.
JAN (Japan Article Number) is a standardized bar code system widely used for general consumer goods. Another application of bar codes is a distribution material symbol. This symbol is added to the JAN code in the form of a distribution identification code of one or two digits.
Any of the above bar code symbols is called a one-dimensional bar code. The amount of information that those code systems can handle is several tens of bytes at most.
Recently, however, there has been a strong demand for a bar code capable of having a greater amount of information. To meet the demand, various symbol systems called two-dimensional bar codes have been proposed.
According to these symbol systems, a remarkably large amount of information can be coded, as compared to one-dimensional bar code systems. Specifically, the amount of information is increased by stacking one-dimensional bar codes. The symbols of these systems are called stacked bar codes. These types of stacked bar codes include a code system called PDF-417.
An example of a conventional symbol information reading apparatus for reading stacked bar codes is a laser scan type apparatus disclosed, e.g. in Published Unexamined Japanese Patent Application (PUJPA) No. 2-268382. According to this apparatus, a laser beam is scanned two-dimensionally, thereby reading and decoding bar code symbol information.
However, the laser type bar code symbol information reading apparatus as disclosed in PUJPA No. 2-268382 has a problem: since scan information is successively analyzed, bar code symbol information cannot exactly be read unless the direction of arranged bar codes on an image region (hereinafter referred to as "label") of a bar code symbol is made substantially parallel to the direction of scanning.
Of course, the same problem lies in the one-dimensional bar code symbol information reading apparatus. In particular, the height of one row of bar codes of the stacked bar code symbol is less than the height of the bar codes of the one-dimensional bar code symbol. Thus, the control of the scan angle is important in the case of the stacked bar code label, and the stacked bar code label needs to be set on the label detection surface of the reading apparatus in the exact direction. Accordingly, the direction of the label must be varied many times until the information is read exactly, and the high-speed information input which characterizes the bar codes cannot be achieved.
Furthermore, PUJPA No. 2-268383 discloses an apparatus wherein bar codes are scanned by a two-dimensional imaging apparatus, an image of the bar codes is stored in a memory, and a bar code symbol information is decoded on the basis of the data stored in the memory.
This bar code symbol information reading apparatus using the two-dimensional imaging apparatus, however, has the same problem as the laser type apparatus: unless the direction of arranged bar codes on the label is set to be substantially parallel to the XY direction of the two-dimensional imaging apparatus, i.e. the direction of address of the image memory, the bar code symbol information cannot exactly be read. Although PUJPA No. 2-268383 states that bar codes can be read even when the direction of the arranged bar codes is not parallel to the XY direction, an actual method for reading is not taught. In addition, this prior art reference is silent on the case where a bar code label is scanned outside the field of vision of the imaging apparatus.